Refrigeration systems are used in many applications for heating and cooling a controlled environment, including homes, buildings and cargo boxes of transport trucks, trains, ships or planes. Before operating a refrigeration system in a cooling or heat/defrost mode, it is desirable to determine whether the refrigeration system 102, is in an operational condition. To determine if the refrigeration system is functioning properly, a diagnostic "pretrip" routine is run to ensure proper operation. Pretrip is a process of testing a refrigeration system before operating the system in a cooling or heat/defrost mode to ensure that the system will operate properly.
To date there have been few attempts to create pretrip testing methods, and those proposed pretrip routines have been inadequate for a variety of reasons. For example, U.S. Pat. No. 5,172,561, discloses a pretrip operational method for automatically checking predetermined functions and the operating performance of a transport refrigeration unit. This system measures the current draw from predetermined electrical components with a prime mover stationary, and compares the measured current draw of those components with an allowable range for the component. When a control unit in the comparison step finds that the current draw for each component checked is in the associated allowable range, the actual speed of the internal combustion engine is determined, and compared to the actual speed of the internal combustion engine to determine if it is in an allowable range. This pretrip method employs temperature sensors which sense the temperatures of the return air, discharge air and ambient air. The system is then commanded to operate in a cooling mode, and the unit is checked for adequate cooling capacity as a function of the temperature values of the return air, discharge air, and ambient air. It is then determined if the transport refrigeration system is operating in the commanded heating mode by comparing the temperatures of the return air and discharge air. Operation of the modulation valve is determined by sensing the speed of the prime mover before and after closing of the modulation valve.
Although this pretrip method attempts to verify whether the system will operate correctly, it suffers from numerous defects. For example, one disadvantage is that the pretrip routine executed is the same regardless of the desired box temperature or ambient temperature. This creates numerous problems especially when heating in warm ambients or cooling in cool ambients. Another limitation of the related art pretrip methods is that they failed to determine the causes of specific problems within the refrigeration system, and even assuming they could determine the cause of the problem, these related art pretrip methods could not determine if problem resulted from the failure of a particular component of the system. Yet another disadvantage of these pretrip routines they only evaluate a change in temperature to determine whether the refrigeration system will function properly. Thus, the related art pretrip methods failed to evaluate whether the system is maintaining necessary pressures at certain points in the system. As a result, alarms in the system would often sound when they were not supposed to (i.e., false failures), or conversely not sound when there were actual problems in the system. Either of these problems is very undesirable. For example, false alarms cause the refrigeration system to be taken out of service and inspected for problems, which takes time and costs money. In addition, a false alarm takes even longer to service because there are no identifiable problems. Furthermore, failing to sound an alarm when there are actual problems in the system often results in destruction of the cargo.